The 2008 bronze medallist outshone team-mate and defending Olympic champion Lu Chunlong to score 62.990.
Russia's Dmitry Ushakov finished in silver medal position, with Lu taking bronze.
Canada's Jason Burnett, who narrowly missed out on gold at the last Games to win silver, hit the edge of the bed after his first skill to finish last in the final.
Dong revealed he has been mentally preparing for the Olympic Games in a special way.
'I've been looking forward to the competition,' Dong said after he collected his medal. 'It's not about the result, it's just one stage in life.
'Just before the Olympics I wrote in a journal everyday and told myself I would do my best in the Games.
'It was one of my dreams to win this medal and I'm happy that the greatest moment has come.'
When asked what today's journal entry would be, Dong said: 'I thought about that yesterday. I think I will write: "Haha".'